This is with reference to the article entitled "Validity of Montreal Cognitive Assessment in Non-English speaking patients with Parkinson's disease" published in Neurol India 2015; 63: 63-7. [1] The present case-control study was designed with the aim to assess the validity of Montreal Cognitive Assessment scale for use in non-English (Malayalam) speaking patients with Parkinson's disease (PD). The authors, at the end of the study, conclude that this study establishes the reliability of cross-cultural adaptation of Montreal Cognitive Assessment for assessing cognition in Malayalam-speaking Parkinson's disease patients, for their early screening and potential future interventions for cognitive dysfunction. [1] Authors deserve appreciation for their efforts. However, I have some concerns with this study.

The authors have a chosen a case-control design for the purpose of this study and as per the authors, 70 patients with Parkinson's disease and 60 age- and education-matched healthy controls were chosen for this purpose. [1] Herein lies my first concern. The authors in the first place have used a smaller number of controls that may not provide adequate power to the conclusion drawn from the study sample. But more importantly, it appears that the authors have not matched for age and education. Age and education matching of controls means that a similar proportion of cases fall into the various categories defined by the matching variable (age and education in this study). For instance, if 25% of the cases are aged 65-75 years, 25% of the controls would be taken to be of the same age.

This takes me to my second concern. The authors aim to assess the validity of Montreal Cognitive Assessment but conclude with a statement on the reliability of cross-cultural adaptation of Montreal Cognitive Assessment for assessing cognition. Validity and reliability are not same. Validity of a test/assessment is the degree to which it measures what it is supposed to measure. This is not the same as reliability. Reliability is the extent to which a measurement gives results that are consistent or repeatable. Furthermore, within validity, the measurement does not always have to be similar. But in reliability, the measurements are similar. Also, the reliability of a measure does not necessarily make it valid (and vice versa).

Lastly, a comment specific to this study; in psychometrics, validity has a particular application known as test validity. Test validity refers to the degree to which evidence and theory support the interpretation of test scores ("as entailed by the proposed uses of the tests"). The validity check in this study should have looked at this aspect.

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